In commercial blasting operations, a series of explosions is frequently triggered in an exact order with precise timing. For this purpose, blasting systems have been developed that employ shock tubes also known as signal transmission lines that transfer a blast initiation signal to a series of explosive charges. To facilitate this, a signal from a single shock tube can be transferred to multiple shock tubes in a blasting system via the use of connector block/detonator assemblies, thereby permitting the initiation of multiple explosive charges in a controlled manner.
Safety and reliability are paramount for any blasting system, and efficient shock tube initiation is an important factor in this regard. Shock tubes that fail to initiate result in unexploded charges at the blast site, with inevitable safety concerns. Moreover, the reliable initiation of shock tubes is imperative to ensure the required blasting pattern is effected.
The design of the connector block has a significant influence upon the efficiency of shock tube initiation. For reliable initiation, sufficient energy must be transferred from the base charge of a detonator to the shock tubes, thus compressing the shock tubes extremely rapidly in order to initiate them. Several connector block designs, are known in the art, which have been developed to improve the efficiency of energy transfer from the base charge of the detonator to the shock tubes.
The most efficient transfer of energy from the detonator base charge to the shock tubes occurs when the surface of the percussion-actuation end of the detonator is in direct contact with the shock tubes. If any gap is present between the detonator end and the shock tubes, the transfer of actuation energy may be less efficient, thus resulting in an increased failure rate of shock tube initiation. However, excess pressure from the percussion-actuation actuation end of the detonator upon the shock tubes can result in the distortion of the shock tubes, and consequently the reduction of shock tube internal volume within the connector block. This in turn reduces the capacity of the shock tubes for efficient initiation, since their capacity for rapid compression is also reduced.
Connector blocks and their components are generally manufactured by plastic molding techniques that are well understood in the art. Quality control during the manufacturing process can ensure a degree of uniformity in the dimensions and mechanical properties of the connector blocks produced. However, slight differences between connector blocks are unavoidable due to tolerances in the plastic resulting from both the manufacturing process, and from the properties of the plastic material. Slight differences may also occur in the dimensions of the detonator. Such tolerances can give rise to improper positioning of the detonator within the connector block, relative to the shock tubes. For example, upon actuation of the detonator, a slight gap between some of the shock tubes and the percussion-actuation end can result in a reduction in energy transfer to the shock tubes.
Therefore, it is desirable to design a connector block wherein the detonator can be securely and optimally positioned to contact but not squeeze the shock tubes within the block. Previously, several attempts have been made to design connector blocks with improved reliability of shock tube initiation. However, it is important to note that previous designs generally involve the use of detonator retention means such as clips, latches, and collar locks to secure the detonator within the block. Typically, such detonator retention means employ elements that are integrally molded into the plastic of the block, or molded as a separate component. For this reason, the position of the detonator within the block is specifically governed by the position of the retention means, which locks the detonator into a fixed position relative to the shock tubes. Therefore, the distance between the retention means and the shock tubes is fixed at the point of manufacture of the connector block, and no allowance is subsequently made for tolerances in the plastic material of the block or the dimensions of the detonator.
In one example of such a device, U.S. Pat. No. 4,815,382 issued Mar. 28, 1989, discloses a connector block comprising a plastic tube having a bore, with at least one transverse bore arranged perpendicular to the main bore. The main bore is designed to receive a detonator shell, and the transverse bores can receive a length of shock tube. The detonator shell may be fixed within the connector block by means of a circumferential lip on the inside wall of the main bore, which engages a circumferential crimp at the percussion-actuation end of the detonator shell. In this way, the detonator is secured within the plastic housing of the connector block.
In another example, corresponding U.S. Pat. Nos. 5,171,935 and 5,398,611 issued Dec. 15, 1992 and Mar. 21, 1995 respectively, disclose a detonator block with a positioning means on the housing of the block, for positioning the detonator in juxtaposed signal transfer relationship with one or more shock tubes. In certain embodiments of the invention, there are also provided deformable tabs within the housing for snap-fit retention of the detonator within the connector block.
Subsequent improvements in connector block design lead to the use of collar locks for detonator retention. For example, U.S. Pat. No. 5,423,263, issued Jun. 13, 1995, discloses a connector block designed for transfer of explosive energy from the detonator for bi-directional initiation of shock tubes. In a preferred embodiment, the detonator may be held in the connector block by a collar lock device that secures the detonator at the closure crimp, present at the end of the detonator opposite the percussion-actuation end. The collar lock is slidably mounted within a groove in the block that runs perpendicular to the longitudinal axis of the detonator.
An alternative design of connector block is disclosed by U.S. Pat. No. 5,499,581, issued Mar. 19, 1996, which comprises an integral slidably mounted locking member. Once the detonator is inserted into the connector block, the locking member is displaced to rupture a frangible web and engage the closure crimp of the detonator. Moreover, the displaced locking member itself becomes locked into the displaced position by engaging the connector block. In an alternative embodiment, various shapes for the locking member are disclosed, each to secure the detonator in a fixed position relative to the shock tubes, and ensure irreversible engagement of the locking member in the displaced position.
An apparent modification to U.S. Pat. No. 5,499,581 is disclosed by U.S. Pat. No. 5,792,975, issued Aug. 11, 1998. In this regard, a similar connector block is provided comprising a slidably mounted locking member. The patent discloses an improvement in the configuration of the locking member, wherein the member comprises at least one wedge-shaped surface, so that upon displacement of the locking member towards its locking position, the wedge-shaped surface moves the detonator axially into position, adjacent to the shock tubes. In this way, the position of the detonator is biased towards the shock tubes.
As will be apparent from the discussion above, the connector blocks of the prior art frequently include complex design features to lock the detonator in a desired position. Moreover, the corresponding manufacturing processes may require several molds to produce the multiple components for the block, followed by the precise assembly of the components. It is undesirable to produce complex connector blocks for several reasons. Design complexity, and the need for multiple manufacturing steps, can result in a reduction in the quality and reliability of the connector blocks. In addition, production costs also increase with design complexity.
For practical use at the detonation site, connector blocks must be robust, reliable, and not prone to failure. The inclusion of intricate features in connector block design such as slidably mounted locking members can be detrimental to ease of handling in the field, as well as the functionality and the robustness of the blocks.
There is therefore a need for connector blocks of improved design and improved methods of manufacture of such blocks.